Is it important to model a car to be subdivided in all quads or will tris work?
When tris get subdivided, they become quads. Use tris in your mesh sparingly. 

 

At regular intervals or when you're finished with modelling, you must check for double points in mesh before unwrapping or rendering. To do that, select entire mesh, press W and remove doubles. Note: Doubles occur when you use mirror modifier. So always remember to remove doubles before ever rendering - otherwise you will have rips in rendered mesh. 

Whenever I get stuck on topology, I always go to Pinterest for ideas - TONS of ideas and sources for inspiration. If you haven't got Pinterest, it is time that you start a new account there. Pinterest is where you pin pictures to your boards: those on topology, node compositing, alpha brushes, etc. 
In this link, my board is called Mesh Studies and I collect others' topology works. I have those for hands, feet, faces, etc. 
https://www.pinterest.ie/ritag2/a-mesh-studies/